This invention relates to a method for fabricating a multiple holographic lens from a fly's eye lens with a single exposure. One example of an application of multiple holographic lenses is to provide for parallel addressing of many discrete matched filter memory elements with a single input laser beam.
Present techniques of fabricating such lenses require multiple superimposed exposures for multiple storage elements. Multiple exposures result in distortion, reduced efficiency, and non-uniformity among channels on playback.
If on the order of a hundred or more elements are to be recorded using multiple exposures, the intensity of the reference and signal beams of each exposure must be reduced, relative to what would be used for a single exposure hologram, to avoid overexposing the film. However, the reduction in recording beam intensity also reduces the intensity of the desired playback or reconstruction beam. Undesired beams may be characterized as cross-product terms resulting from the phasor multiplication of the many holographically stored beams in the recording of the lens. The cross product terms also reduce the efficiency of the lens since a larger proportion of the incident beam on reconstruction is diverted to undesired refraction beams.
Further inefficiency results due to an inability to use the proper biasing value of the film's characteristic curve. This is because the cumulative intensity of the multiple exposures fogs; i.e., biases the film toward the saturation of non-linear end of the film's characteristic curve, resulting in distortion and nonuniformity of beams on playback.
A final deficiency in the present techniques of fabricating multiple holographic lenses is that there is no way to select both the f-number and the focal length of the completed lens. Thus, either the f-number is determined by a selected focal length, or the focal length follows from a selected f-number.